According to the commonly quoted statistics, less than 5% of the population donates blood regularly. And with concerns of West Nile and Mad Cow and other diseases that may or may not be able to be transmitted through blood donations, the percentage of people eligible to donate is going down, too. Right now, about 60% of the population is eligible.
I know some people have belonephobia (fear of needles), hemaphobia (fear of blood), Iatrophobia (fear of doctors), nephophobia (fear of disease), and other such phobias that keep them away from the blood bank. But the societal benefits of having a ready supply of available are so great and it's really easy to do. Plus, you get a quick blood pressure check, and there are indications (source: LifeSouth Community Blood Centers commentary [PDF]) that regularly donating blood can reduce the risk of heart attacks, strokes, and other "cardiovascular events" for non-smoking men. Also: "Preliminary studies involving 6,500 men were conducted by Kansas University Medical Center and The University of Kuopio [Kuopion Yliopisto] in Finland. Researchers believe that by giving blood, men and post-menopausal women rid their bodies of excess iron, which is thought to contribute to heart disease."
Two reasons I try to donate regularly is that I am type O-, which makes my blood particularly helpful, and Evelin can't donate anymore because she spent too much time in England during and after college (mad cow concerns have led to deferrals for people who've spent a cumulative total of three months in the United Kingdom since 1980 or a total of six months in any European country or combination of countries since 1980).
It can be given to persons with any blood type and it is needed for emergency room patients where there may not be time to determine blood type. O- is also the preferred blood type for newborns if they need a transfusion.
The Blood Bank of Alaska has a chart of personality traits associated with different blood types.
Type O: You want to be a leader, and when you see something you want, you keep striving until you achieve your goal. You are a trendsetter, loyal, passionate, and self-confident. Your weaknesses include vanity and jealously and a tendency to be too competitive.Like any horoscope or similar thing, there are bits that make me go "Of course that's me," but other bits just sound silly.